Complex electronic components, particularly large scale integrated circuits (LSI's) such as microprocessors, have many internal interconnections where voltages cannot be directly measured except by microprobes. These microprobes are unsatisfactory because they sometimes damage the component or introduce unwanted electrical loading. Voltage contrast images have been obtained utilizing a scanning electron microscope (SEM) whose output video signals contain information relating to voltages at these interconnections. However, these images quickly become difficult or impossible to interpret as component complexity increases. There has long been a need for a non-destructive method and apparatus to compare internal voltages of an operating electronic component such as the type above-described and a reference electronic component in which differences between internal voltages at corresponding points on the two components can be rapidly identified in order that appropriate isolation or corrective measures may be taken. The present invention provides such a method and apparatus.